Project Summary/Abstract The long-term objectives of this proposal are to identify how TREM2 regulates microglial functions and tau pathology, and how this is affected by the R47H mutation. TREM2 is a receptor expressed by microglia in the healthy brain. A heterozygous R47H mutation in TREM2 (R47H-TREM2) significantly increases risk for late- onset Alzheimer's disease (AD); however, the effects of this mutant TREM2 on microglial functions are unknown. In addition, the field has yet to address whether TREM2 modulates tau spread, the phenomenon whereby tau pathology appears in a stereotypical pattern as AD progresses. Based on published findings and my preliminary data, the PI hypothesizes that while both trem2 haploinsufficiency and R47H-TREM2 impair microglial phagocytosis and motility through loss-of-function mechanisms, R47H-TREM2 exacerbates tau spread through gain-of-toxic functions. To test this hypothesis, the PI will carry out two aims using novel human TREM2 knock-in mouse models that the PI generated. Aim 1: the PI will identify the functional effects of trem2+/- and TREM2R47H/+ on microglia. The PI will use primary microglia to assess cell-autonomous phagocytosis of different substrates. The PI will also use acute brain slices to determine whether trem2+/- and TREM2R47H/+ affect phagocytosis in an age- and brain region-dependent manner. Finally, she will characterize TREM2's role in microglial motility using in vivo two-photon imaging of microglia at baseline and upon injury in trem2+/- and TREM2R47H/+ mice at young and old age. Aim 2: the PI will assess whether trem2+/- and TREM2R47H/+ affect the spread of tau fibrils and spread-associated pathology in mice using histology. Moreover, she will use RNA-sequencing to characterize differentially expressed genes between TREM2WT/+ and TREM2R47H/+ microglia. The PI will then manipulate these genes in primary microglia to determine the relevance of TREM2-mediated microglial functions to tau spread and to attempt to ameliorate tau spread. The proposed research is significant because it will address the functional consequences of trem2 haploinsufficiency and R47H-TREM2 on microglia and how this affects tau pathology, highlighting microglial pathways with therapeutic promise. The PI will establish a novel role for TREM2 as a regulator of microglial motility, a critical function that has not been studied in this field, while also addressing the age-dependency of TREM2-mediated functions given its association with late-onset AD. In addition to AD, these results may be relevant to other diseases associated with aberrant microglial activity and tau pathology, as the pathways identified in this study are likely to be affected in these diseases as well.